XIAN, China, June 30 (Yonhap) -- The just-ended state visit to China by South Korean President Park Geun-hye offers one of the best promises yet to upgrade relations with a nation that has the greatest leverage over North Korea and trades heavily with the South.

The four-day trip also strengthened Park's personal bonds with Chinese President Xi Jinping as they spent a combined seven and a half hours together, talking and dining from Thursday's summit and a welcoming reception to Friday's special lunch Xi hosted in an unusual show of hospitality.

In a large part, the visit was an effort by Park to shore up relations with an important neighbor amid widespread perceptions that South Korea, under her predecessor, former President Lee Myung-bak, put too much emphasis on strengthening the alliance with the United States.

Throughout the trip, Park tried to show how much she cares about China, sometimes lacing her remarks and speeches with phrases and sentences in the Chinese language. Officials said Park studied Chinese by herself and has a relatively good command of the language.

"Only about 20 years have passed since South Korea and China established diplomatic relations in 1992, but friendship and cooperation have developed at a pace nearly unprecedented in the world," Park said during a speech at Beijing's Tsinghua University.

"I believe that Korea-China relations should now move forward into a more mature and substantial partnership ... I intend to pursue dialogue and cooperation in a more forward-moving way based on the deep trust forged with President Xi through the summit," she said.

On Saturday, Park also offered to return 360 sets of remains of Chinese troops killed in the 1950-53 Korean War, a symbolic gesture of friendship toward the former battlefield foe. During the war, China fought alongside North Korea against the U.S.-backed Allied Forces.

Park made the proposal during a meeting with Chinese Vice Premier Liu Yandong right before her speech at Tsinghua University, the alma mater of Chinese President Xi. Liu expressed gratitude, saying the offer well conveys Park's friendship toward China.

The highlight of the visit was Thursday's summit with Xi.

After the meeting, the two sides issued a wide-ranging joint communique pledging to bolster cooperation in all sectors, including political and security areas. Also adopted was an appendix that lays out a string of specific cooperation commitments in a move aimed at ensuring the communique won't be empty words.

In an effort to bolster political and security cooperation, the two sides agreed to establish a dialogue channel between South Korea's presidential national security chief and China's state councilor in charge of foreign affairs. They also agreed in principle on setting up a hotline between their foreign ministers.

The agreement was highly meaningful given that Seoul and Beijing have made strides in their economic relations since the 1992 establishment of diplomatic relations, but their political and security relations have not moved forward enough to match the economic ties.

That is in large part because of disagreements over how to deal with North Korea.

The summit was also watched closely as to how much support Park would win from China in pressuring Pyongyang to give up its nuclear programs because it took place as Beijing has been taking an unusually tough stance on the North after its December rocket launch and February nuclear test.

According to the communique, the two sides agreed that North Korea's nuclear program poses a serious threat to regional and world peace, and pledged to work closely together to make the Korean Peninsula free of atomic weapons.

But the document stopped short of clearly stating that a nuclear North Korea is unacceptable.

As a key provider of economic aid and diplomatic protection for North Korea, China has long been considered the only country with any meaningful influence over Pyongyang, though it has been reluctant to use that leverage over concern that pushing the North too hard could hurt its national interests.

But some analysts say that China could be shifting its policy focus on Pyongyang as the prospect of a nuclear North Korea has become a valid concern following February's atomic test.

Since the nuclear test, China has backed a U.N. sanctions resolution against the North and has been carrying out the restrictions more vigorously than before. Beijing even joined separate American sanctions by suspending all transactions with the North's Foreign Trade Bank, which was accused of financing Pyongyang's nuclear programs.

But others say China's recent toughness does not represent any fundamental change in policy.

They say China still places top priority on keeping Pyongyang alive because instability in the neighboring nation could hurt its economic growth, trigger a massive influx of refugees and lead to the emergence of a unified Korean Peninsula under South Korean and U.S. control across its border.

On economic issues, the two sides agreed to step up efforts to expand bilateral trade volume to US$300 billion by 2015 and make progress in ongoing negotiations to free up trade between two of Asia's biggest economies, according to the communique.

Last year's trade volume between the two sides amounted to $215 billion.

The two countries also agreed to extend their currency swap deal by three years to keep it valid until 2017. South Korea and China agreed in 2011 to double their won-yuan swap line to 360 billion yuan. That accord was to expire in 2014.

On the second leg of the trip, Park made a visit to the ancient Chinese city of Xian.

Xian, an ancient capital with more than 3,000 years of history, is a base for China's push to develop western parts of the country and has great potential for economic cooperation as the city could serve as a foothold for South Korean firms trying to expand into Central Asia and Europe.

Park discussed economic cooperation with Zhao Zhengyong, the Communist Party chief of the province.